All methods of stereoscopic pick-up require that a stereo-pair of two images of a scene be picked up from two slightly different horizontal angles. At the display the two images are superimposed and one image is channeled to each respective eye of the viewer. Historically, one of the first methods of recording and displaying stereoscopic, i.e., three dimensional (3D), pictures to a group of viewers was the use of two film cameras to record the two images and two slide projectors to display the stereoscopic picture. Color coding was employed to channel the two images separately to the viewer's eyes by anaglyph (two-color) projection and with corresponding anaglyph (two-color) glasses.
For example, a black-and-white photographic slide of the left image of a recorded stereo-pair is placed in the left projector and a slide of the right image of the stereo-pair is placed in the right projector. A red filter is placed in front of the lens of the left projector and a green filter is placed in front of the lens of the right projector. The two projectors are angled so that their images superimpose on the screen.
Without glasses, the viewer sees the object of principal interest in the scene completely superimposed and it presents therefore, a nearly black-and-white picture because the intensities and hues of the red and green projectors are adjusted so the mixture produces whites and grays. Things behind the object of principal interest are not completely superimposed nor are those in front of the object of interest. This causes a color fringe of red or green to occur on the edges of these objects. Narrow objects in the background or foreground such as posts may appear separated as one red post and one green post. When the viewer places the anaglyph glasses in front of his eyes, with a red filter in the left aperture and a green filter in the right aperture, each eye sees only one image of the stereo-pair. The left eye sees the red image of the left slide through the red filter in the left aperture of the glasses. The green light from the right slide is blocked and absorbed by the red filter. The right eye sees the green image of the right slide through the green filter in the right aperature of the glasses. The red light from the left slide is blocked and absorbed by the green filter. The brain fuses the two channeled images into a single three dimensional picture which appears in all areas to be black-and-white.
Other combinations of filters may be used, such as red and blue, green and magenta (red-blue), yellow (red-green) and blue, and red and cyan (blue-green). Each combination has certain advantages and disadvantages. Generally, red and cyan have been found to be the most desirable colors for anaglyph viewing.
Another configuration is to use an enlarger stand to project the two photographic black-and-white slide images, one at a time, through a red filter for the left eye and a green filter for the right eye and record them superimposed on color film. This color film may be projected by a single projector and viewed with two-color anaglyph glasses as a black-and-white three dimensional picture.
Generally, the stereo-pair of slides use black-and-white film with the color being added artificially by filters at the display and in the glasses for color coding to thereby channel one image to each eye. Each of the two black-and-white film slides records a similar shade of gray in identical areas of the picture. However, if a red filter is used in front of one black-and-white film camera and a cyan filter in front of another, the two slides will differ in shades of gray in identical areas causing the projected picture to appear in color and in three dimensions.
That is, a reddish area of the scene may be recorded as light gray in the left slide and nearly black in the right slide appearing therefore, reddish to the viewer. A cyan area may be recorded as black in the left slide and white in the right slide. A yellow area may be recorded as medium gray in each slide. Corresponding color filters at the projector add color back in the scene. However, in this configuration blue and green are linked together as cyan and it is not possible to record, display, or view green or blue separately or in their varying shades.